The present invention relates to a method for surface priming in adhesive bonding or, more particularly, to a method for surface priming in adhesively bonding a metal and a silicone rubber by use of an organosilicon compound as a priming material.
As is well known, silicone rubbers are widely used in a variety of applications by virtue of their inherently excellent physical and chemical properties and it is not rare that silicone rubbers are used as a part of a composite body with other parts of a metal or plastic resin by adhesively bonding together. One of the problems in the preparation of such a composite body is the poor adhesive bonding strength between a silicone rubber and a metal or plastic resin. Therefore, it is a usually undertaken method that the adhesive bonding of a silicone rubber composition to the surface of a metallic or plastic part is preceded by coating the surface with a primer composition composed of an alkoxysilane compound having an aliphatically unsaturated linkage in the molecule such as vinyl trimethoxysilane, vinyl tri(2-methoxyethoxy)silane, 3-methacryloxypropyl trimethoxysilane and the like or a hydrolysis condensation product or cohydrolysis polysiloxane thereof, a titanate ester and a metal salt of a carboxylic acid. The priming effect obtained by use of the above described primer composition is, however, not strong enough so that the adhesive bonding strength cannot be prolongedly maintained when the adhesively bonded composite body is heated at a temperature of 200.degree. C. or higher. Futher, the adhesion between these materials has a relatively low resistance against oils at an elevated temperature so that a phenomenon of peeling or sep-aration sometimes takes place at the interface of adhesion when an adhesively bonded composite body is dipped in an oil at a high temperature.
Accordingly, a proposal has been made in Japanese Patent Publication No. 56-39817 for an improved primer composition of this type having a rubber cement-like consistency as composed of a vinyl-containing organopolysiloxane, an organopolysiloxane resin, an alkoxysilane, an organic peroxide and a catalyst. Although this primer composition is effective to improve the adhesive bonding strength at a high temperature in a dry condition or in an oil, the preparation thereof is performed disadvantageously taking a great deal of time and labor due to the complicated formulation thereof. Further, when the primer composition is applied to the injection molding of a silicone rubber compound, the primer layer may be partly or wholly peeled away by the flow of the rubber compound. In addition, certain troublesomeness is unavoidable in the practical use of the primer composition of this type due to the poor air-dryability and the excessive flowability which necessitates preheating of the primer layer to ensure evenness of the primer coating.